Ugh. Google has made the process of using multi-page forms much harder than it used to be. In the older version of Google Forms the values were passed from one page to another in an array.
In the new version of Google Forms the data is still passed from page to page in a variable (called draftResponses) however PHP sees this data as a string instead of an array so the code which reformats the array from a PHP style (which WordPress uses) to a Python style (which Google uses) doesn’t run.
It looks like I will need to parse the string into a PHP array and then encode the array to be compatible with Python. As I said, ugh.